This change adds a truncate_bytes mode which counts the number of bytes, as
opposed to the number of UTF-8 characters. This can be useful for ensuring that a
crypt-encoded filename will not exceed the underlying backend's length limits
(see https://forum.rclone.org/t/any-clear-file-name-length-when-using-crypt/36930 ).
This change also adds support for _keep_extension when using truncate and
truncate_bytes.
Before this change the help for convmv was generated by running the
examples each time rclone started up. Unfortunately this involved
running the echo command which did not work on Windows.
This pre-generates the help into `transform.md` and embeds it. It can
be re-generated with `go generate` which is a better solution.
See: https://forum.rclone.org/t/invoke-of-1-70-0-complains-of-echo-not-found/51618
